Stars are classified based on the spectral type (i.e. a means to measure the photospheric temperature and density by getting information about the ionisation state).
Under the Morgan–Keenan (MK) system using the letters O, B, A, F, G, K, and M, a sequence from the hottest (O type) to the coolest (M type) (Harvard Spectral classification based on the surface temperature of the stars). A luminosity class is added to the spectral class using Roman numerals (Yerkes Spectral classification). This is based on the width of certain absorption lines in the star's spectrum (0 or Ia+ - hypergiants, I - supergiants, II - bright giants, III - regular giants, IV - sub-giants, V - main-sequence stars, sd - sub-dwarfs, and D - white dwarfs).